1. Field of the Invention
The present invention generally relates to an optical signal transmission system, and more particularly to an optical signal transmission system employing a novel circuit in a pulsed frequency modulation system.
2. Description of the Prior Art
In short-haul television transmission for subscribing systems and cable television systems (referred to as "CATVs"), a light source is directly intensity-modulated in response to an input signal in order to easily transmit analog information such as an image (picture) or speech through an optical signal transmission line such as optical fiber cables. In such transmission systems, however, modulation distortion occurs due to poor linearity of the light source itself, resulting in a degradation of transmission quality. To achieve high-quality signal transmission with a relatively simple scheme, the following system is expected. An analog signal is pre-modulated by an electronic circuit into a pulse signal. Light from the light source is then intensity-modulated in response to this pulse modulated signal. Such modulation systems have been proposed in the form of PWM (pulse width modulation system), PDM (pulse duration modulation system), PFM (pulse frequency modulation system) and pulsed frequency modulation system. Among these systems, the pulsed FM system is preferred for increasing utilization efficiency of the transmission frequency band. This system has many advantages in that multiple transmission can be easily performed, S/N (signal-to-noise) ratio is improved, and offset DC components can be eliminated by AC coupling means in the reception side.
In principle, in the pulsed FM system, the FM-modulated analog signal is processed in a predetermined way to obtain a pulsed FM signal. This pulsed FM system differs from the PFM (pulse frequency modulation system) in that pulse width as well as pulse frequency are changed in response to the level of the analog signal to be transmitted. Although a modulator capable of a large frequency shift must be employed in order to obtain a pulsed FM signal, it is conventionally difficult to easily produce such a modulator. A voltage-controlled multivibrator (VCM) may be considered as one means for obtaining a pulsed FM signal. In general, when input voltage is set relatively low, the voltage-controlled multivibrator is subjected to a degradation of linearity in voltage-frequency characteristics due to the non-linearity of the transistors constituting this multivibrator. As a result, modulation distortion occurs and the modulation factor (.DELTA.f/f0) is insufficiently high. Since it is also difficult to regulate the voltage-frequency and capacitance characteristics in this case, the number of components must be necessarily increased. Accordingly, indispensable requirements, i.e., simplification of transmission systems, to subscribing systems and CATVs cannot be satisfied.
Furthermore, since the pulsed FM signal does not include baseband signal components, it precludes the use of a simple demodulator in the reception side. For example, basic spectral components of the pulsed FM signal are extracted by a band-pass filter and are demodulated by an analog signal processing method. This type of analog signal processing is normally employed by normal FM radio receivers as such a modulation system using a phase locked loop (PLL). When a pulsed FM signal with a large frequency shift is demodulated by the analog FM demodulator, the number of circuit elements must be necessarily increased in order to improve the linearity of the frequency-voltage conversion characteristics.
On the other hand, when noises (e.g., sinusoidal noises) correlated with the time, but not with random noises, are mixed in the receiver unit, a pulse phase or position of a pulse having a predetermined width is changed in proportion to a differential of applied correlated noises as a function of time. A change in pulse position results in variations in output from a low-pass filter. In particular, the time-correlated noises mixed in the receiver unit are known as modal noises when a laser diode is used as a light source in the receiver unit and a multi-mode type fiber is used.
It is therefore an object of the present invention to provide an optical signal transmission system which overcomes problems of the nonlinearity, noise and circuit arrangement limitations of the conventional optical transmission system.
Another object of the present invention is to provide an optical signal transmission system which highly improves modulation distortions.
A still further object of the present invention is to provide an optical signal transmission system which produces a high quality pulsed FM signal with a simple ciruit arrangement.